Couple of options
accusys makes a variety of internal and external RAID devices that can be connected via FireWire, USB, or SATA.
These are hardware based RAIDs, for no particular OS or software is needed. They come in RAID 0, 1, and 5 configurations, so depending what your pocketbook allows, you can choose.
If you want something a bit cheaper, you can get RAID5 software from
FWB, or use the RAID 0 or RAID 1 features in OSX. These all will cause a minor to major performance hit on your CPU depending on what processor you use.
Finally, if you are looking just for cases ( SATA / USB / FireWire ), I have had particularly good luck with cases from
bytecc .
These have been pretty reliable, and stack well. Pricewatch.com caries the FW versions for about $45.
I hope this helps,
Max.